1. Field of the Invention
This invention relates to a sputtering target and its manufacturing method especially suitable for application to an alloy target used for sputtering in a manufacturing process of a magneto-optical recording medium.
2. Description of the Related Art
For years, price strategies have been important in the field of AV recording mediums such as mini discs (MDs) intended for general customers, and reduction of prices has been demanded continuously. To meet the demand for price reduction of mediums, reduction of manufacturing costs of mediums has been required. Regarding cost reduction of mediums, thickness of targets used for fabrication of recording materials of mediums is an important factor.
That is, for manufacturing a medium, magnetron sputtering, among others, is used for stacking its recording material. For deposition of the recording medium by magnetron sputtering, a target containing the recording material is used. Such a target includes as its major component a rare earth element which is a rare element and expensive. Conventionally, however, only 30% to 50% of the target was sputtered and contributed to deposition of the film, and the remainder of the target was abandoned. Therefore, it has been demanded to increase the ratio of a target actually sputtered and contributing deposition of a film by increasing the thickness of the target.
However, targets conventionally used for manufacturing mediums had magnetic permeability around 5. Therefore, the maximum thickness of a target enabling stable discharge of plasma and sputtering was about 8 mm.
Under the circumstances, various researches and developments have been made toward fabrication of targets having a thickness of 8 mm or more, and various proposals were presented.
For example, there was proposes a technique for manufacturing a target made by hot-pressing alloy powder obtained by mechanical crushing and having a magnetic permeability not exceeding 3 (Japanese Patent Laid-Open Publication No. hei-10-251847 (Literature 1)). Literature 1 teaches a method for manufacturing a magneto-optical recording alloy target made by hot-pressing alloy powder made by mechanical crushing and having a magnetic permeability not exceeding 3, and a method for manufacturing a magneto-optical recording alloy target made by mechanically crushing used targets into alloy powder and mixing it with new alloy powder and having a magnetic permeability not exceeding 3.
Literature 1 also teaches that a target having a low magnetic permeability, low containment of oxygen, single-sintered structure of a rare earth metal and a transition metal, and a thickness not less than 8 mm can be made by dissolving a source material and used targets, then making a quickly cooled alloy, and mechanically crushing the quickly cooled alloy into alloy powder.
However, the Inventor conducted various experiments concerning alloy powder as taught by Literature 1, and found that the alloy powder described in Literature contained an unacceptably large amount of metal impurities for practical use.
For example, there was proposed a technique for manufacturing a target made by hot-pressing alloy powder obtained by mechanical crushing and having a magnetic permeability not exceeding 3 (Japanese Patent Laid-Open Publication No. hei-10-251847 (Literature 1)). Literature 1 teaches a method for manufacturing a magneto-optical recording alloy target made by hot-pressing alloy powder made by mechanical crushing and having a magnetic permeability not exceeding 3, and a method for manufacturing a magneto-optical recording alloy target made by mechanically crushing used targets into alloy powder and mixing it with new alloy powder and having a magnetic permeability not exceeding 3.
The magneto-optical recording alloy target having a magnetic permeability not larger than 3, which is disclosed in Literature 1, also involves the problem that a sufficient leak magnetic flux cannot be obtained in any magnetron sputtering apparatus, and sputtering of this target is impossible.
That is, although the Inventor made a target having the magnetic permeability of 2.1 and the thickness of 10 mm from alloy powder as taught by Literature 1, and tried sputtering by setting this target in place of a sputtering apparatus, it could not sputter this target.
Furthermore, although Literature 1 does not teach a magneto-optical recording alloy target having a magnetic permeability not larger than 2.3, according to the Inventor""s knowledge, a 10 mm thick target having a magnetic permeability around 2.3 as taught by Literature 1 cannot be sputtered.
It is therefore an object of the invention to provide a sputtering target and its manufacturing method enabling effective use of expensive rare earth metals, not adversely affecting the environment, contributing to reduction of the manufacturing cost, and ensuring a target with a magnetic permeability low enough for sputtering.
The Inventor made researches toward solution of the above-indicated problems involved in the conventional techniques. A summary thereof is shown below.
According to the Inventor""s knowledge, in order to effective use an expensive rare earth metal for fabricating a target whose thickness is not less than 10 mm, it is desirable to produce recycled alloy powder by using used targets, and fabricate new targets by using the recycled alloy powder. So, the Inventor conducted various experiments regarding rare earth alloy power containing recycled alloy powder.
The Inventor first made reviews about theoretic density ratios of targets fabricated. According to the Inventor""s knowledge, rare earth alloy powder is very readily oxidized, and if a target substantially made of rare earth alloy powder has a low theoretic density ratio, oxidation of the target itself progresses. Therefore, any medium made by using this target cannot have a satisfactory property. Relation between deposition rate in a sputtering process and target lifetime is shown in FIG. 1, taking two different cases where the theoretic density ratio of the target is 95% and 97%, respectively.
It is apparent from FIG. 1 that, when the theoretic density ratio is high, i.e. 97%, fluctuation in deposition rate of sputtering is small. In other words, if thickness of the target is constant, the target lifetime is longer, and the manufacturing cost of mediums can be reduced when the theoretic density ratio is high.
Therefore, it is preferable for a target to have a theoretic density ratio not lower than 95%, and more preferably not lower than 97%. Discussion is continued below, selecting the case of the theoretic density ratio not lower than 97%.
For the purpose of determining composition of rare earth alloy powder, the Inventor conducted an experiment about dependency of magnetic permeability of rare earth alloy powder upon quantity of rare earth metals contained in rare earth alloy powder. A result of the experiment is shown in FIG. 2.
It apparent from FIG. 2 that magnetic permeability of rare earth alloy powder is 5 or higher when it contains about 20 weight % of rare earth metals, but decreases to 2 or lower when the content of earth metals therein is 35 weight % or higher. Therefore, in order to maintain a magnetic permeability not higher than 2 in rare earth alloy powder, the content of rare earth metal in the rare earth alloy powder is preferably not lower than 35 weight %, and more preferably not lower than 40 weight %.
The Inventor also conducted an experiment about magnetic permeability and theoretic density ratio of targets upon rare earth alloy powder contained in alloy powder, using targets prepared by using rare earth alloy powder. A result of the experiment is shown in FIG. 3. In FIG. 3, values of magnetic permeability of targets are plotted with ▪ whereas values of theoretic density ratio are plotted with ∘.
It is apparent from FIG. 3 that permeability of targets is larger than 2 when the content of rare earth alloy powder in alloy powder is less than 65 weight %, and becomes 2 or less when the content of the rare earth alloy powder in alloy powder is not less than 65 weight %, or preferably not less than 70 weight %, taking errors into account.
Thus, content of rare earth alloy powder in alloy powder as the source material of a target should be not less than 65 weight %, and more preferably not less than 70 weight %.
The Inventor also conducted an experiment about magnetic permeability and theoretic density ratio of targets. FIG. 4 shows dependency of magnetic permeability and theoretic density ratio of targets upon content of rare earth alloy powder in alloy powder, when magnetic permeability of the rare earth alloy powder is controlled not to exceed 2 (more specifically around 1.5). In FIG. 4, values of magnetic permeability of targets are plotted with ▪ whereas values of theoretic density ratio are plotted with ∘.
It is apparent from FIG. 4 that magnetic permeability of targets becomes larger than 2 when amount of rare earth alloy powder, having a magnetic permeability not larger than 2, contained in alloy powder used as the source material of targets is less than 50 weight %, and becomes 2 or less when 50 weight % or more.
Therefore, content of rare earth alloy powder having a magnetic permeability not higher than 2 in alloy powder used as the source material of targets is preferably controlled not to be lower than 50 weight %.
Through those various experiments and researches, the Inventor has come to know that, in order to control magnetic permeability of a target not to exceed 2 while using recycled alloy powder, it is necessary to control the content of rare earth alloy powder including recycled alloy powder contained in alloy powder used as the source material of the target not to exceed 65 weight %.
The invention has been made through those researches and accompanying experiments.
According to the first aspect of the invention, there is provided a sputtering target made of alloy powder which contains at least 65 weight percent of at least one kind of rare earth alloy powder made of at least one kind of rare earth element and at least two kinds of elements selected from the group consisting of Fe, Co, Ni, Cr and Si, and contains at least one kind of recycled alloy powder prepared by using a target used at least once for sputtering.
In the first aspect of the invention, magnetic permeability of the target is typically a value not larger than 2.
In the first aspect of the invention, thickness of the target ensuring a sufficient leak magnetic flux intensity and enabling sputtering is typically not less than 8 mm and not more than 20 mm, or preferably not less than 10 mm and not more than 15 mm.
According to the second aspect of the invention, there is provided a manufacturing method of a sputtering target which manufactures a target from alloy powder which contains at least 65 weight percent of at least one kind of rare earth alloy powder made of at least one kind of rare earth element and at least two kinds of elements selected from the group consisting of Fe, Co, Ni, Cr and Si, and contains at least one kind of recycled alloy powder prepared by using a target used at least once for sputtering.
In the second aspect of the invention, for the purpose of ensuring a practical level of the content of metal impurities in the alloy powder used for making the target, the recycled alloy powder is prepared by powdering by an atomizing method an ingot prepared by a target used at least once for sputtering and a material not used before for making a target. From the standpoint of making fine powder minimized in composition segregation, typically used as the atomizing method is a gas atomizing method. However, a single-roll method or a centrifugal disc method is also usable.
In the present invention, recycled alloy powder is a substance obtained by powdering an ingot prepared from a target used at least once for sputtering, and a new material not used before for making a target. In the present invention, recycled alloy powder typically contains rare earth alloy powder prepared from a target used at least once for sputtering by 30 weight % or more, or preferably by 50 weight % or more.
In the present invention, a rare earth element is the generic name of lanthanoids, Sc (scandium) and Y (yttrium) to lanthanoids. More specifically, it is the general name of La (lanthanum, Ce (cerium), Pr (praseodymium), Nd (neodymium), Pm (promethium), Sm (samarium), Eu (europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), Lu (lutetium), Y and Sc.
In the present invention, alloy powder typically contains 50 weight % or more of rare earth alloy powder having a magnetic permeability not larger than 2. In order to control the magnetic permeability of the rare earth alloy powder not to exceed 2, content of rare earth metal in the rare earth alloy powder is typically not less than 30 weight %, and preferably not less than 35 weight %.
In the present invention, in order to make a target having a magnetic permeability not larger than 2, magnetic permeability of alloy powder is controlled not to exceed 2.
In the present invention, content of metal impurities in the target is typically not more than 0.1 weight %, and content of metal impurities in the alloy powder is not more than 0.1 weight %.
In the present invention, theoretic density ratio of the target is typically not less than 97%, and preferably not less than 98%.
In the present invention, the sputtering apparatus using the target is typically a magnetron sputtering apparatus. However, the target can be made in any other sputtering apparatus, such as opposed-electrodes sputtering apparatus, electron cyclotron resonance (ECR) sputtering apparatus, high-frequency sputtering apparatus, reactive sputtering apparatus, bias sputtering apparatus, collimate sputtering apparatus or long-distance (LD) sputtering apparatus.
In the present invention, the sputtering target is preferably made of alloy powder which contains at least one kind of rare earth alloy powder selected from the group consisting of FeTbCo, FeTbCr, FeTbCoCr, FeGdCo, FeDyCo and FeGdCoSi by 65 weight % or more, and containing at least one kind of recycled alloy powder prepared by using a target which is made of at least one kind of rare earth alloy selected from the group consisting of FeTbCo, FeTbCr, FeTbCoCr, FeGdCo, FeDyCo and FeGdCoSi used at least once for sputtering.
According to the sputtering target and its manufacturing method having the above summarized structures according to the invention, since the sputtering target is made of alloy powder which contains at least one kind of rare earth alloy powder made of at least one kind of rare earth element and at least two kinds of elements selected from the group consisting of Fe, Co, Ni, Cr and Si, and contains at least one kind of recycled alloy powder prepared from a target used at least once for sputtering, the sputtering target can be lowered in magnetic permeability, and increased in thickness.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.